1. Field of the Invention
The present invention relates to a pneumatic tire having a sipe cut into a land portion formed in a tread surface, and more particularly to a pneumatic tire useful as a studless tire.
2. Description of the Related Art
Generally, a studless tire has a sipe cut into a rib or a block as a land portion formed in a tread surface, and thus such configuration increases an edge effect of a sipe edge and a water removing effect of the sipe to increase ice performance (braking performance on an icy road).
However, in the pneumatic tire having a sipe, a load applied to the land portion may cause the sipe to be easily closed to prevent a sufficient water removing effect from being provided. With increasing wear of the land portion, the height of the land piece surrounded by the Sipes is reduced to increase rigidity. This may cause an insufficient inclination of the land piece and reduce the edge effect.
In order to prevent an insufficient increase in ice performance due to the abovementioned insufficient water removing effect, a method for reducing a sipe width (sipe thickness) and increasing the number of sipes to be formed in the land portion may be supposed. This allows the water removing effect to be covered by the edge effect to generally increase the ice performance.
Further, in order to increase an edge effect at the time of wearing of a land portion, a pneumatic tire is known in which a waveform sipe is formed, and the number of waves of the sipe is gradually increased toward a sipe groove bottom as disclosed in Japanese Patent Laid-Open No. 2004-161166. The above-described configuration can increase an edge effect at the time of wearing of a land portion to increase ice performance for problems.
As described above, the above-described solutions may be effectively combined to increase the ice performance of the pneumatic tire having a sipe throughout a period of use. Specifically, it may be effective that the waveform sipe is formed, the number of waves of the sipe is gradually increased toward the sipe groove bottom, and the sipe width (sipe thickness) is reduced to increase the number of sipes to be formed in the land portion.
However, in the tire having the above-described configuration, a sipe density is increased to reduce rigidity of each land piece. Also, a sipe blade for forming the sipe has waveforms on its opposite surfaces, which increases the surface area. Then, when a tire after vulcanization is released from a vulcanizing oven, vulcanization problems can rise such as insufficient release of the tire with a part of the land piece kept in tight contact with the sipe blade or generation of a surface crack. Rigidity of the sipe blade itself is also reduced, which results in reduction of a useable life of the sipe blade.
In order to increase a water removing effect, as disclosed in Japanese Patent Laid-Open No. 7-276923, a tire is also known in which a recess (wide sipe portion) is formed in a sipe inner wall surface, and water is introduced into the recess. However, in the tire described in Japanese Patent Laid-Open No. 7-276923, the forming of many recesses in a sipe inner wall surface may reduce rigidity of a land piece.
Thus, in view of the above-described problems, the present invention has an object to provide a pneumatic tire that can exhibit high ice performance throughout a period of use of the tire, and can prevent vulcanization problems such as insufficient release of the tire or a surface crack.